This invention relates to band pass filters and more particularly to surface acoustic wave (SAW) filters to provide flexible electrical filtering functions in the 10 MHz to 1 GHz frequency range.
Although very good filters have been produced in the 10 MHz to 1 GHz frequency range, there is a need to increase attenuation in the rejection band and to cut the ripple in the passband, while at the same time reducing the substrate area of the SAW device to minimize cost. See, E. M. Garber and H. A. Haus (1983), "Synthesis of High Performance SAW Filters with Nonuniformly Spaced Fingers", Proceedings IEEE 1983 Ultrasonics Symposium, pp. 27-32.
SAW filters consist of two transducers, an input transducer which converts an electrical signal into an acoustic one, and an output transducer which converts it back again. Filtering is accomplished in the electro-acoustic conversion process. SAW filters utilize a piezoelectric material. The electrical signal excites surface acoustic waves via the piezoelectric effect. Physically, each transducer consists of two interdigitated metal combs deposited on a piezoelectric substrate. An example of a prior art interdigital transducer is shown in FIG. 1. A transducer element 10 includes first and second metal combs 12 and 14. The fingers 16 of the comb 12 are interdigitated with the fingers 18 of the metal comb 14. The acoustic sources of such a structure can be approximately localized to the gaps between fingers attached to opposite bus bars 20 and 22 of the combs 12 and 14. The space between acoustic sources can be simply treated as a time delay due to the dispersionless nature of SAW propagation. The impulse response of a transducer 10 roughly consists of impulses at the gaps. The time domain response of the entire filter is the convolution of the responses of the two transducers comprising the filter, or the product of their Fourier Transforms in the frequency domain. See, C. S. Hartmann, D. T. Bell, Jr., and R. C. Rosenfeld (1973), "Impulse Model Design of Acoustic Surface Wave Filters", IEEE Trans. on Sonics and Ultrasonics, SU-20, pp. 80-93.
At the present time, high performance frequency responses typically require apodized impulse responses. Apodized impulse responses are implemented by varying the overlap between fingers attached to opposite bus bars of a comb structure. Such a prior art transducer element is shown in FIG. 2. In this case, a transducer 30 includes comb structures 32 and 34 whose fingers overlap by varying amounts. The problem with the transducer element 30 is that diffraction and transverse electrostatic end effects from the many small overlaps typically required for high performance filters severely degrade the frequency response and are difficult to compensate for in practice. See, W. R. Mader, C. Ruppel and E. Ehrman-Falkenau (1982), "Universal Method for Compensation of SAW Diffraction and Other Second Order Effects", Proceedings IEEE 1982 Ultrasonics Symposium, pp. 23-28 and R. S. Wagers (1976), "Transverse Electrostatic End Effects in Interdigital Transducers", Proceedings IEEE 1976 Ultrasonics Symposium, pp. 536-539.
Withdrawal weighting was introduced to avoid the above-mentioned problem with apodization. See, C. S. Hartmann (1973), "Weighting Interdigital Surface Wave Transducers by Selective Withdrawal of Electrodes", Proceedings IEEE 1973 Ultrasonics Symposium, pp. 423-426. In this technique, a prototype amplitude modulated waveform is approximated by a sequence of constant amplitude samples and some zero weight ones. Since the aperture is uniform, diffraction and electrostatic end effects are much less significant, and the maximum aperture may even be reduced to save substrate area and thereby cut costs. Generally speaking, the aperture is the distance between the bus bars. The withdrawal weighting technique is based on the fact that in a narrow band sense, acoustic source weights can be moved by half a wavelength and reversed in sign, without affecting the frequency response significantly. The currently known withdrawal weighted configuration is shown in FIG. 3 and is referred to as Sturcture I. A withdrawal weighted transducer 40 includes comb structures 42 and 44 having fingers which create gaps having either 1, 0, or -1 weights as shown in the figure. With a structure such as 40, it is a geometrical necessity that the non-zero gap weights alternate in sign. This is a rather strong restriction. For example, the non-implementable sequence of weights 1,0,1,0 would represent in a withdrawal weighting sense the frequently needed sequence of half strength samples, 1/2,-1/2,1/2,-1/2. Using the structure 40, such a sequence would have to be represented as -1,1,0, 0,-1,1,0,0 which is clearly a worse approximation since averaging must be done over four samples instead of only two to see that half weight samples are being approximated.
It is therefore an object of this invention to provide a surface acoustic wave transducer element which overcomes the problems of both apodized and standard withdrawal weighted filter structures.
It is a further object of the invention to provide such a transducer element which facilitates the design of sharp cut-off filters with low pass band ripple and low sidelobes over a broad frequency range.
Another object of the invention is a transducer element which is fabricated by standard integrated circuit fabrication techniques.